The mechanism is intended for a vehicle having an automatic or semi automatic gearbox where the ability to remove a key inserted in the mechanism is governed by the position of the gear shifter of the gearbox. In addition, the ability to move the gear shifter from a given position, usually the park position, may depend upon the brake pedal being depressed. Systems already exist in which the ignition key switch mechanism, brake pedal and gear shifter are interlinked so that an inserted key may only be removed under certain conditions. One such known arrangement is described in U.S. Pat. No. 5,562,568. In this arrangement an interlock system is provided for a vehicle brake, gearbox gear shifter and ignition key switch. The interlock system comprises control means having two bi-stable solenoids one of which is operative to lock and unlock the gear shifter in park position and the other of which is operative to block and unblock ignition key movement to the off position for removal. It is an object of the invention to simplify such arrangements.